An intensified charge coupled image sensor comprises an image intensifier tube having a photoemissive cathode on an interior surface of an input window and a charge coupled device (CCD) located at the focal plane of the image intensifier tube. Such a structure is shown in U.S. Pat. No. 4,355,229 issued to H.S. Zimmerman et al, on Oct. 19, 1982. The Zimmerman et al. patent utilizes a CCD imager commercially available from RCA Corporation, Lancaster, Pa., and designated as the SID 52501. The CCD of the Zimmerman et al. patent, shown in FIGS. 1 and 2, includes an image array, known as an A register, a temporary storage array, known as a B register, and an output register, known as a C register. The B and C registers are usully masked, that is, means (not shown) are provided for preventing photoelectrons from the photoemissive cathode from reaching either register.
The A and B registers have channel stops (not shown) extending in the column direction to isolate the channels (the columns of the CCD) from one another. The electrodes (shown in FIG. 2) may be of the single layer type comprising, for example, N+ type regions of polysilicon separated by P-type regions of polysilicon. These electrodes extend in the row direction and, in the example illustrated, are three-phase operated. The electrodes are insulated from the relatively thick P-type substrate by a layer of silicon dioxide (SiO.sub.2). The SID imager mentioned above has 320 columns and 512 rows (256 in the A register and 256 in the B register), each row comprising a group of three electrodes.
The operation of the CCD of FIG. 1 is well understood. During the so-called integration time, a scene or other image is projected onto the A register. The light or other radiant energy of the image causes charges to be produced at the various locations of the A register in accordance with the light intensity or energy density reaching the respective locations.
Upon the completion of the integration time (e.g., during the vertical blanking interval of commercial television), the charge signals which have accumulated (a "field") are transferred, in parallel, in the column direction from the A to the B register by the application of the multiple phase voltages .phi..sub.A1 . . . .phi..sub.A3 and .phi..sub.B1 . . . .phi..sub.B3. The charges subsequently are transferred a row at a time, from the B register to the C register, and after each row of charges reaches the C register, it is serially shifted out of the C register in response to the shift voltages .phi..sub.C1 . . . .phi..sub.C3. The transfer of charges from the B to the C register occurs during a relatively short time (the horizontal blanking time of commercial television, which is about 10 .mu.s) and the serial shifting of the C register occurs at relatively high speed (during the horizontal line display time of commercial television). During the transfer of a field from the B to the C register, a new field may be integrated in the A register.
For a black and white CCD imager, the illumination of the A register occurs through the single layer electrode structure. However, if one desired to sense color information, then illumination in this way is not entirely suitable because the electrodes have high absorption at the blue end of the spectrum. For such a CCD imager, it is necessary to thin the substrate to a thickness of about 10.mu. (microns) and to illuminate through the back surface of the thinned substrate. When a CCD is used in an intensified charge coupled image sensor, it is also thinned to minimize lateral dispersion of the charges produced by the incident photoelectrons.
In one practical approach to thinning substrates, the number of CCD imager devices produced on a wafer is relatively small. First, using a two-inch wafer, up to three such devices, each about 0.5".times.0.8", as shown in FIG. 1, are fabricated at the same time on a common substrate, employing photolithographic techniques. Then two of the three devices are masked, that is, except for the back surface of the device being thinned, the entire wafer is coated with a substance (a "resist") which is not attacked by the chemical bath (an acid) used to thin the substrate. Then the entire wafer is immersed in the thinning bath and the wafer is spun about an axis passing through the center of the device being thinned. After the desired amount of thinning of the device is obtained, the wafer is removed from the bath, the resist is removed, and then the masking and other processing steps are repeated for each additional device. Then the wafer is cut apart in such a way that there is a thick substrate border surrounding the thin region of each device. This thick border provides some stiffness and mechanical support for the relatively fragile thinned substrate region of the A register (see FIG. 3).
While the process above has resulted in the production of many operational CCD's, it is not without its problems. One problem is that it is difficult to obtain uniform thinning throughout the entire imaging portion (A register) of the device. It is thought that because of the rectangular shape of the device, the acid bath sometime does not attack as strongly some of the edge and corner regions of the device as the center of the device and these edges and corners therefore are sometimes thicker in the final product than the center regions of the device. Such nonuniformity is undesirable as it sometimes causes nonuniformities in the picture information produced by the CCD. Also, as a practical matter, one cannot manufacture at the same time a large number of devices on the same wafer, even a large wafer. If one were to employ a larger wafer say 4" or 5" in diameter, there would be problems in spinning the wafer during thinning about an axis considerably displaced from the center of the wafer and therefore it would be difficult to utilize the outer edge portion of the wafer (recall that the axis about which the spinning takes place passes through the center of the region being thinned). In addition, the yield using this method is not as high as desired. Also, because of the fragility of the thinned substrate, it is very difficult to test the devices after they are thinned. The reason is that the test probes tend to cause the thinned substrate to become broken or otherwise damaged.
U.S. Pat. No. 4,266,334, issued to T. W. Edwards, et al., on May 12, 1981, and incorporated by reference herein for purpose of disclosure, describes a method which permits numerous thinned CCD imagers to be made on a single large wafer and then to be easily tested, cut out and mounted with little risk of breakage. In the Edwards et al. patent, a relatively large silicon wafer with proper doping is processed in the conventional way, using photolithographic techniques, to produce a relatively large number of CCD's on the common thick substrate. The front surface (the surface containing the electrodes) of the wafer may now be masked, for example, by placing it in a special masking fixture, or by employing a substance, such as a resist. The masking fixture or substance is made to cover the front surface (the one containing the electrodes) of the wafer and the peripheral edge of the back surface of the wafer. Now instead of thinning the individual devices one at a time, as in the previous process described above, the entire wafer is thinned in a rotary etching bath to the desired thickness over its entire center area, leaving only an unthinned rim around the peripheral edge of the wafer, for support, as shown in FIG. 4. Then a sheet of glass which fits into the thinned region is glued to the back surface, as shown in FIG. 5, to provide a laminated structure which is strong and rigid. The individual devices are then separated from one another by cutting through the glass and thinned substrate along lines between imagers.
The CCD's thinned by this method cannot be used directly in intensified charge coupled image sensors because the adhesive used to glue the sheet of glass into the thinned region of the wafer adversely affects the formation of the photoemissive cathode within the image intensifier tube, and the glass sheet and adhesive attentuate the electrons emitted from the photoemissive cathode preventing the electrons from entering the CCD. It is therefore necessary to remove the glass and adhesive from the device after testing and before the device can be mounted within the image intensifier tube. To remove the glass, the wafer containing a number of CCD's is placed into a holder which is shown in FIG. 6 and which prevents the electrode side of the CCD's from contacting the etchant. A combination of H.sub.2 SO.sub.4 and HF acids are put into the top section of the holder to etch the glass sheet. The CCD's are then sectioned from the wafer. The CCD, having a thickness of about 10 micrometers is fragile and therefore difficult to handle and mount in the image sensor. The Zimmerman et al. patent discloses that support springs are used to contact the CCD and retain it against a suitable CCD holder plate; however, the mechanical contacts provided by the Zimmerman et al. structure are unreliable and sometimes causes damage to the CCD. Also, the high temperature bakeout of the image sensor prior to the formation of the photoemissive cathode frequently causes the CCD to break because of the thermal mismatch between the CCD and the metal holder plate and contacts.